Streptococcus pneumoniae has been one of the most extensively studied microorganisms since its first isolation in 1881. It was the object of many investigations that led to important scientific discoveries. In 1928, Griffith observed that when heat-killed encapsulated pneumococci and live strains constitutively lacking any capsule were concomitantly injected into mice, the nonencapsulated could be converted into encapsulated pneumococci with the same capsular type as the heat-killed strain. Years later, the nature of this “transforming principle,” or carrier of genetic information, was shown to be DNA. (Avery, O. T., et al., J. Exp. Med., 79:137-157 (1944)).
In spite of the vast number of publications on S. pneumoniae many questions about its virulence are still unanswered, and this pathogen remains a major causative agent of serious human disease, especially community-acquired pneumonia (Johnston, R. B., et al., Rev. Infect. Dis. 13(Suppl. 6):S509-517 (1991)). In addition, in developing countries, the pneumococcus is responsible for the death of a large number of children under the age of 5 years from pneumococcal pneumonia. The incidence of pneumococcal disease is highest in infants under 2 years of age and in people over 60 years of age. Pneumococci are the second most frequent cause (after Haemophilus influenzae type b) of bacterial meningitis and otitis media in children. With the recent introduction of conjugate vaccines for H. influenza type b, pneumococcal meningitis is likely to become increasingly prominent. S. pneumoniae is the most important etiologic agent of community-acquired pneumonia in adults and is the second most common cause of bacterial meningitis behind Neisseria meningitidis. 
The antibiotic generally prescribed to treat S. pneumoniae is benzylpenicillin, although resistance to this and to other antibiotics is found occasionally. Pneumococcal resistance to penicillin results from mutations in its penicillin-binding proteins. In uncomplicated pneumococcal pneumonia caused by a sensitive strain, treatment with penicillin is usually successful unless started too late. Erythromycin or clindamycin can be used to treat pneumonia in patients hypersensitive to penicillin, but resistant strains to these drugs exist. Broad spectrum antibiotics (e.g., the tetracyclines) may also be effective, although tetracycline-resistant strains are not rare. In spite of the availability of antibiotics, the mortality of pneumococcal bacteremia in the last four decades has remained stable between 25 and 29%. (Gillespie, S. H., et al., J. Med Microbiol. 28:237-248 (1989).
S. pneumoniae is carried in the upper respiratory tract by many healthy individuals. It has been suggested that attachment of pneumococci is mediated by a disaccharide receptor on fibronectin, present on human pharyngeal epithelial cells. (Anderson, B. J., et al., J. Immunol. 142:246-2468 (1989). The mechanisms by which pneumococci translocate from the nasopharynx to the lung, thereby causing pneumonia, or migrate to the blood, giving rise to bacteremia or septicemia, are poorly understood. (Johnston, R. B., et al., Rev. Infect. Dis. 13(Suppl. 6):S509-517 (1991).
Various proteins have been suggested to be involved in the pathogenicity of S. pneumoniae, however, only a few of them have actually been confirmed as virulence factors. Pneumococci produce an IgA1 protease that might interfere with host defense at mucosal surfaces. (Kornfield, S. J., et al., Rev. Inf. Dis. 3:521-534 (1981). S. pneumoniae also produces neuramimidase, an enzyme that may facilitate attachment to epithelial cells by cleaving sialic acid from the host glycolipids and gangliosides. Partially purified neuramimidase was observed to induce meningitis-like symptoms in mice; however, the reliability of this finding has been questioned because the neuramimidase preparations used were probably contaminated with cell wall products. Other pneumococcal proteins besides neuramimidase are involved in the adhesion of pneumococci to epithelial and endothelial cells. These pneumococcal proteins have as yet not been identified. Recently, Cundell et al., reported that peptide permeases can modulate pneumococcal adherence to epithelial and endothelial cells. It was, however, unclear whether these permeases function directly as adhesions or whether they enhance adherence by modulating the expression of pneumococcal adhesions. (DeVelasco, E. A., et al., Micro. Rev. 59:591-603 (1995). A better understanding of the virulence factors determining its pathogenicity will need to be developed to cope with the devastating effects of pneumococcal disease in humans.
Ironically, despite the prominent role of S. pneumoniae in the discovery of DNA, little is known about the molecular genetics of the organism. The S. pneumoniae genome consists of one circular, covalently closed, double-stranded DNA and a collection of so-called variable accessory elements, such as prophages, plasmids, transposons and the like. Most physical characteristics and almost all of the genes of S. pneumoniae are unknown. Among the few that have been identified, most have not been physically mapped or characterized in detail. Only a few genes of this organism have been sequenced. (See, for instance current versions of GENBANK and other nucleic acid databases, and references that relate to the genome of S. pneumoniae such as those set out elsewhere herein.) Identification of in vivo-expressed, and broadly protective, antigens of S. pneumoniae has remained elusive.